1. Field of the Invention
This invention relates to a system for estimating air/fuel ratio of an internal combustion engine, more particularly to a system for estimating air/fuel ratios in the individual cylinders of a multicylinder internal combustion engine with high accuracy.
2. Description of the Prior Art
It is a common practice to install an air/fuel ratio sensor at the exhaust system confluence point of a multicylinder internal combustion engine to detect the air/fuel ratio at that location. A system of this type is taught by Japanese Laid-Open Patent Application No. Sho 59(1984)-101,562, for example. Aside from the above, the assignee earlier proposed, in Japanese Laid-open Patent Application No. Hei 5-180,059 which was filed in the United States under the number of 07/997,769 now abandoned, designing a mathematical model describing the behavior of the exhaust system of a multicylinder internal combustion engine which can estimate air/fuel ratios in individual cylinders from the output of a single air/fuel ratio sensor disposed at the exhaust system confluence point through an observer. The sensor used there is not an O.sub.2 sensor which produces an inverted output only in the vicinity of the stoichiometric air/fuel ratio, but a wide-range air/fuel ratio sensor which produces an output proportional to the oxygen concentration of oxygen in the exhaust gas.
With that arrangement, it became possible to estimate the air/fuel ratios in the individual cylinders with high accuracy. Depending on the operating condition of the engine, however, it may arise a situation where it would be difficult to ensure the calculation time or where the air/fuel ratio sensor response would be inadequate.